Some metals, such as iron, cobalt, copper and zinc are beneficial to organisms in very low concentrations. But excess amounts of these metals may be detrimental and eventually toxic to organisms, even when their concentration is still low. Other heavy metals, such as mercury, lead, and cadmium, have no known nutritional benefit to organisms, and bioaccumulations of such heavy metals cause adverse health effects.
Heavy metals, such as mercury and lead for example, cannot be metabolized by the body and if accumulated in the body, cause toxic effects by interfering with physiological functions. Yet heavy metals are found everywhere in the environment because they are naturally part of the earth's crust. Moreover, metallic elements, such as cadmium, chromium, copper, lead, mercury, nickel, and zinc are used to manufacture products and are present in some industrial, municipal, and urban runoff as they are used in coatings, paints, lacquers for packaging, toys, furniture, etc. They do not break down or decompose and tend to build up in plants, animals, and people. Thus, they enter living organisms via food, drinking water, and air. Further, removal of heavy metals from organisms is conventionally a difficult process.